1. Field of the Invention
This invention relates to multitubular heat exchangers and more particularly to a multitubular heat exchanger of the type used in a power generating plant and the like as a feedwater heater handling large volumes of steam and wet steam.
2. Description of the Prior Art
This type of heat exchangers generally comprises a shell, a heating medium inlet formed in the shell, a high temperature tube bundle and a low temperature tube bundle located within the shell and formed by a large number of U-shaped heat transfer tubes for permitting a medium to be heated to flow therethrough so that heat exchange may take place between the medium to be heated and a heating medium introduced into the interior of the shell through the heating medium inlet, and a vent tube interposed between the high temperature tube bundle and the low temperature tube bundle and arranged in the longitudinal direction of the shell for removing noncondensable gas from the interior of the shell. In the aforesaid type of heat exchanger, the ratio of the heat exchanged in the low temperature tube bundle or the tube bundle located near the inlet for medium to be heated to the amount of heat exchanged in the high temperature tube bundle or the tube bundle located near the heated medium outlet has hitherto been about 15:1. This difference in the amount of heat exchanged has given rise to the problem that noncondensable gas of the heating medium accumulates and a stagnating zone of the noncondensable gas is created in the central or deep portion of the low temperature tube bundle in which terminal ends of streams of the heating medium are mainly concentrated. The presence of the noncondensable gas stagnating zone has the disadvantage that it constitutes a portion of the heat exchanger wherein no heat exchange takes place and greatly reduces the performance of the heat exchanger. Also, the noncondensable gas in the noncondensable gas stagnating zone cannot be removed from the interior of the shell through the vent tube interposed between the high and low tube bundles to avoid trouble, so that the presence of the noncondensable gas stagnating zone has the disadvantage of causing corrosion of the heat transfer tubes to occur.
As one of the measures to cope with the above-mentioned problems, Japanese Laid-Open Patent Publication No. 27705/78 laid open for public inspection on Mar. 13, 1978 (this patent publication corresponding to the U.S. patent application, Ser. No. 823,655) proposes to provide at least one flow guide plate on the outer periphery side of at least the low temperature tube bundle for inducing streams of the heating medium flowing between the shell and the tube bundles to pass on to the vent tube. The flow guide plate provided in the prior art as described hereinabove is intended to facilitate the removal of the stagnating noncondensable gas through the vent tube by specifically reducing the size of the noncondensable gas stagnating zone formed in the central portion of the low temperature tube bundle and at the same time moving the zone toward the vent tube.
However, the provision of the flow guide plate has been found to be unable to achieve the meritorious effect of avoiding formation of the noncondensable gas stagnating zone when a heat exchanger is of a large capacity and its tube bundles are of large dimensions.